1. Field of the Invention
This invention relates to a print medium feed system for feeding a print medium for an image forming unit, such as a printer, copying machine, and facsimile.
2. Description of the Related Art
A print medium feed system is provided in an image forming unit, such as a printer, copying machine, and facsimile, to feed a print medium, such as paper, to a print section. When a print medium is fed to the print section from the print medium feed system, it is possible that the print medium has been skewed. If the print medium is fed to the print section in a skewed condition, the print medium is transported in the skewed condition in the print section too. Under the condition, print is skewed on the print medium, which results in poor printing quality of the image forming unit. Accordingly, a slip roller is rotatably provided in front of the print section to correct the skew of the print medium.
FIG. 3 is a top view of a conventional print medium feed system, showing the operation thereof. FIG. 2 is a sectional view of FIG. 3 taken along with the line A—A, showing a main part of the print medium feed system.
In FIGS. 2 and 3, reference numeral 21 denotes a print head and 22 is a platen, which is rotatably provided and opposed to the print head 21. A print section P1 is provided between the print head 21 and the platen 22. Reference numeral 23 is a first guide for guiding a print medium 13, 24 is a second guide which is spaced from the first guide 23 at a predetermined distance and guides the print medium 13. A transport route Art is formed between the first and second guides 23 and 24 to transport the print medium 13 to the print section P1.
A pair of feed rollers 12 are rotatably provided on the upstream side of the print section P1 in a transport direction of the print medium 13. The print medium 13 is transported by rotating the feed rollers 12. Each of the feed rollers 12 is composed of a shaft 12a and a plurality of roller members 12b provided on the shaft 12a in the axial direction of the shaft 12a. A plurality of table sensors 14 are provided under a table 16 on the upstream side of the feed rollers 12 in the transport direction the print medium 13. A plurality of skew sensors 15 are provided under the table 16 on the downstream side of the feed rollers 12.
A slip roller 11 is provided on the upstream side of the feed rollers 12 and the table sensors 14 in the transport direction of the print medium 13. The slip roller 11 is rotatably provided on the side of the second guide 24 with respect to the transport route Rt and faces the transport route Rt. The slip roller 11 is composed of a shaft 11a and a plurality of roller members 11b provided on the shaft 11a in the axial direction of the shaft 11a. 
When the print medium 13 is set on the table 16 such that the print medium 13 is overlapped with at least one of the roller members 11b, the table sensors 14 detect the set print medium 13 and sends a detection signal to a control section (not shown).
When the control section reads the detection signal, it drives a skew correction motor (not shown), rotates the slip roller, and transports the print medium 13. At this point, the feed rollers are stopped.
As the print medium 13 is transported, the front end (an upper side in FIG. 3) of the print medium 13 abuts against a contact point of a certain roller member 12b′ of the feed rollers 12. The respective roller members 11b are disposed at positions corresponding to those of the respective roller members 12b. Accordingly, when the front end of the print medium 13 abuts against the contact point of the certain roller member 12b′, a roller member 11b′ corresponding to the roller 12b′ slips with respect to the print medium 13 afterwards. Consequently, the roller member 11b′ does not transport the print medium 13. At this point, the roller members 11b other than the roller member 11b′ continue the transportation of the print medium 13.
When the front end of the print medium 13 abuts against contacts of the respective roller members 12b one after another, the respective roller members 11b corresponding to the roller members 12b slip with respect to the print medium 13. Accordingly, the respective roller members 11b stop transporting the print medium 13.
As described above, the skew of the print medium is corrected. The roller members 11b are made of a flexible rubber material to slip sufficiently with respect to the print medium 13. Also, the pressure of the slip roller 11 against the print medium is adjusted.
After the sip roller 11 rotates at a predetermined travelling amount, which is more than the distance between the slip roller 11 and the feed roller 12, the control section sends a drive signal to a line feed motor (not shown) to drive the line feed motor.
After the skew correction is finished, the feed rollers 12 transport the print medium 13. As the print medium 13 is transported, the front end of the print medium 13 arrives at the skew sensors 15. The respective skew sensors 15 detect the arrival and send medium detection signals to the control section. The control section reads in the respective detection signals, detects the difference in detection times on right and left sides of the print medium, and judges if the difference in detection time exceeds a threshold. When the difference exceeds the threshold, the control section judges that the skew has not been corrected and drives the line feed motor in the opposite direction. Then, the feed rollers 12 are rotated in the opposite direction to expel the print medium 13 in the left-hand direction in FIG. 2.
When the difference is less than the threshold, the control section judges that the skew has been corrected and continues driving the line feed motor. Consequently, the feed rollers 12 are rotated in the feed direction so that the print medium 13 is supplied to the print section P1. Then, print is performed in the print section P1.
In the conventional print medium feed system, however, as shown in FIG. 6, when the front end of the print medium 13 is curled, even if the front end 13b of the print medium 13 is pressed to the feed roller 12 by the rotation of the slip roller 11, the front end 13b does not abuts against a contact point 19 of the roller member 12b occasionally. That is, the front end 13b of the print medium 13 is transported upwardly along the roller members 12b instead of abutting against them. Consequently, the transportation of the print medium 13 fails so that the jam of the print medium occurs. Also, the correction of the skew is not performed surely.